Antibody-drug conjugate (ADC) technology is a target-oriented technology, which allows for selective apoptosis of cancer cells. Typically, ADCs function by targeting cancer cells using the antibody and then releasing a toxic material (i.e., the drug) in a cell, thereby triggering cell death. Since ADC technology allows a drug to be accurately delivered to a target cancer cell and released under specific conditions, while minimizing collateral damage to healthy cells, ADC technology increases the efficacy of a therapeutic antibody and decreases the risk of an adverse reaction.
A basic structure of an antibody-drug conjugate is an “antibody-linker-low molecular drug (toxin)”. The linker ideally allows the drug to exhibit an effect on a target cancer cell, e.g., after being separated from the antibody (for example, by enzyme-mediated hydrolysis), after the drug reaches a target cell. The linker also plays a functional role, by connecting the antibody and the drug. The efficacy and toxicity of the antibody-drug conjugate thereby depends, in part, on the stability of the linker, and thus, the linker plays an important role in drug safety.
The linkers of antibody-drug conjugates may be roughly classified as non-cleavable or cleavable. Many non-cleavable linkers are attached to antibodies using a thioether, comprising a cysteine of the antibody. The pendant drug generally cannot dissociate from the antibody in vivo. In the case of the widely-used thiol-maleimide method, however, the antibody-drug conjugate is unstable, which may result in dissociation of the drug from the conjugate before or after it reaches a target cell.
Cleavable linkers may be hydrolyzed, for example, by a cytosolic enzyme. A cleavable linker may comprise a disulfide bond, e.g., including a cysteine of the antibody. A disulfide linker, which allows for dissociation via a thiol exchange reaction, relies in part on the uptake of an antibody-drug conjugate into a target cell and the exposure of the disulfide to the cytosol, which is a reducing environment. Since various types of thiols (for example, albumin, and glutathione) are present in the blood, however, a drug may dissociate from the antibody prior to reaching its target.
In light of the foregoing, improved linkers for antibody-drug conjugates are desirable.